Slim, lightweight notebook computers (laptops) have been recently developed more and more. With this, there have been increasing demands on further slimmed and further sophisticated protective films for polarizers (polarizer-protecting films) used in display devices such as liquid crystal display devices. Liquid crystal display devices display an image (information) through the control of polarization by a liquid crystal and thereby require polarizers. Stretched films of a poly(vinyl alcohol) (PVA) containing iodine are generally used as the polarizers. The polarizers are fragile, and, for protecting them, polarizer-protecting films are used. In general, triacetylcellulose films have been widely employed as the polarizer-protecting films. Independently from such polarizer-protecting films, retardation films are used in such optical devices for controlling the phase difference (retardation) of polarized light. Retardation films adopted typically to the liquid crystal display devices are used in combination with polarizers to solve problems such as color compensation and viewing angle extension by the use of thickness-direction retardation Rth. Some of them have the function of converting linearly polarized light to circularly polarized light or, vice vista, converting circularly polarized light to linearly polarized light using an in-plane retardation Re, at all wavelengths in the visible light range.
Such polarizer-protecting films are provided in order to protecting polarizers, and cellulose acetate films are most desirably used as the polarizer-protecting films in consideration of production process of the polarizers, because the polarizers to be protected are composed of a PVA containing moisture. In contrast, materials other than cellulose acetates have been used for retardation films, so as to exhibit optical properties. Specifically, exemplary materials for retardation films customarily used include polycarbonates, polysulfones, poly(ether sulfone)s, and amorphous polyolefins. The films made from these polymers characteristically have a retardation decreasing with an increasing wavelength of light and thereby exhibit ideal retardation properties at all wavelengths in the visible light range.
To convert linearly polarized light to circularly polarized light or, vice versa, to convert circularly polarized light to linearly polarized light by the action of one retardation film in the visible light range, the retardation film preferably show a retardation of one fourth the wavelength (λ/4) at an incident wavelength (λ) of the retardation film. A retardation film of this type can give a reflective display device with excellent image quality by using the retardation film having a retardation of λ/4 (quarter wave plate) in combination with only one polarizer in a reflective liquid crystal display device having a back electrode serving also as a reflecting electrode. This retardation film is also used as a backside layer of a guest-host liquid crystal layer with respect to a viewer and used as an element for converting circularly polarized light to linearly polarized light in a reflective polarizer which is composed typically of a cholesteric liquid crystal reflecting only one of left-handed and right-handed circularly polarized lights.
The customary retardation films (made typically of polycarbonate (PC), polysulfone (PSu), or PA) have a retardation property of a decreasing in-plane retardation (Re) with an increasing wavelength and are difficult to exhibit ideal retardation properties at all wavelengths in the visible light range. When used in a wide wavelength band, two or more different retardation films are laminated to obtain required performance. To obtain such performance by one retardation film, the retardation film preferably shows an in-plane retardation Re of λ/4 at an incident light wavelength entering the retardation film. For this configuration, the retardation film should have a retardation property of an increasing in-plane retardation Re with an increasing wavelength, contrarily to the above-mentioned retardation property. A cellulose acetate film, if having the retardation property just mentioned above, is useful, because the film can serve both as a polarizer-protecting film and a retardation film, and this eliminates the need for a composite retardation film including two or more different retardation films and allows the optical film (retardation film) to have an improved total light transmission in a liquid crystal display device.
As a possible solution to this problem, Patent Literature (PTL) 1 proposes a technique of using, as a retardation film, an oriented film of a cellulose acetate having a total degree of substitution (degree of acetylation) of from 2.5 to 2.8. The patent literature mentions that, the retardation film according to this technique shows an increasing retardation with an increasing wavelength and exhibits ideal retardation properties at all wavelengths in the visible light range. Specifically, the technique disclosed in PTL 1 provides a retarder (retardation film) which is composed of one retardation film and shows a decreasing retardation with a decreasing wavelength to be measured. An object of this technique is to provide a retarder (retardation plate) which is composed of a polymer oriented film having an increasing birefringence Δn with an increasing wavelength in the range of from 400 to 700 nm, in which the polymer oriented film has an increasing average refractive index with a decreasing wavelength in the wavelength range. As a solution, PTL 1 discloses a technique of stretching and thereby orienting a film of cellulose acetate having a degree of acetylation of from 2.5 to 2.8.
PTL 1 discloses in a working example that a cellulose acetate having a degree of acetylation of 2.661 was prepared in the following manner. Specifically, the cellulose acetate was obtained by dissolving 100 parts by weight of a cellulose triacetate being available from Wako Pure Chemical Industries, Ltd. and having a limiting viscosity [η] of 1.335 and a degree of acetylation of 2.917 in 500 parts by weight of methylene chloride; adding thereto 1000 parts by weight of a 96% aqueous solution of acetic acid; hydrolyzing the cellulose triacetate with acetic acid and water at 70° C. for 100 minutes while removing methylene chloride under reduced pressure; precipitating and washing the reaction product with large excess of water; and drying the washed precipitates. PTL 1 further discloses that 100 parts by weight of the resulting polymer and 3 parts by weight of dibutyl phthalate as a plasticizer were dissolved in 700 parts by weight of a 9:1 (by weight) solvent mixture of methylene chloride and methanol to give a solution, the solution was cast into a film by solution casting, and the film was further uniaxially stretched at a temperature of 170° C. to a draw ratio of 1.5 times. Specifically, PTL 1 mentions in Example 1 that a retardation film having such wavelength characteristics (wavelength dispersion characteristics) as to have an increasing retardation with an increasing wavelength was obtained by stretching. This literature further discloses that the resulting retardation film can also serve as a quarter wave (λ/4) retardation film or another retardation film by regulating its in-plane retardation Re. In addition, PTL 1 discloses a cellulose acetate having a degree of acetylation of 2.421 in Example 4. However, when a film was prepared using this cellulose acetate, and retardation properties of the film were measured, the film showed an insufficient in-plane retardation Re at a film thickness of around 100 μm (from 50 to 150 μm) which thickness is suitable as a freestanding film. Independently, when having a large thickness of around 200 μm, the film showed a suitable in-plane retardation Re of about 80 to 150 nm but an excessively large thickness-direction retardation Rth of more than 350 nm; indicating that the film, when working as a quarter wave (λ/4) retardation film, does not sufficiently work as a viewing angle extension film. In addition, PTL 1 does not describe the molecular weight distribution of the resulting cellulose acetate and neither describes nor indicates the control of retardation properties by controlling the molecular weight distribution.
PTL 2 discloses a cellulose ester film including a cellulose ester which contains an acyl group having 2 to 4 carbon atoms as a substituent, has a total degree of acyl substitution of less than 2.67 at the 2-, 3-, and 6-positions in a glucose residue, and has a degree of acyl substitution of less than 0.87 at the 6-position (Abstract). This technique has been made in order to provide a cellulose ester film, a long-length retardation film, an optical film, and a method for producing the same, in which the cellulose ester film can give, with good productivity, an optical film having a uniform retardation function, showing excellent plane quality (with less pressed concave defects and less thickness deviation), having an easily controllable retardation parameter R0, and showing uniform retardation properties; and to provide a polarizer and a display device each using them and having excellent display quality. PTL 2 describes that the cellulose acetate may be obtained in the following manner. To 100 parts by mass of a cellulose were added 16 parts by mass of sulfuric acid, 260 parts by mass of acetic anhydride, and 420 parts by mass of acetic acid; the mixture was raised in temperature from room temperature to 60° C. over 60 minutes with stirring and subjected to an acetylation reaction for 15 minutes while maintaining the temperature at 60° C.; a solution of magnesium acetate in a mixture of acetic acid and water was then added thereto to neutralize sulfuric acid; water vapor was introduced into the reaction system; the temperature was held at 60° C. for 120 minutes to perform saponification/aging; the resulting substance was then washed with a large amount of water until acetic acid odor was not detected, followed by drying, and thereby yielded a cellulose acetate 1 having a degree of acetyl substitution of 2.65 and a viscosity-average degree of polymerization of 290. PTL 2 discloses that the obtained cellulose acetate 1 had a degree of acetyl substitution at the 6-position of 0.85 as determined by 13C-NMR and that the resulting cellulose acetates obtained in the working examples each had a ratio Mn/Mw (this ratio is probably Mw/Mn) between the number-average molecular weight (Mn) and the weight-average molecular weight (Mw) of from 2.0 (Example 1) to 3.0 (Example 2).
PTL 3 discloses a method for producing a cellulose ester film, which method includes the steps of casting a dope onto a belt support according to a solution casting filming process to form a web; beginning stretching of the web in a machine direction (MD) at the time when the amount of residual solvent in the web is 40 percent by mass or more; and stretching the web in a transverse direction (TD) at the time when the amount of residual solvent in the web becomes less than 40 percent by mass, in which the dope contains an ultraviolet absorber, two or more plasticizers, and a cellulose ester, one of the plasticizers is a polyhydric alcohol ester plasticizer, at least one of the other plasticizer(s) is a plasticizer selected from plasticizers other than phosphoric ester plasticizers, and the cellulose ester has a weight-average molecular weight Mw and a number-average molecular weight Mn with a ratio Mw/Mn of 1.8 to 3.0. This technique has been developed to provide a cellulose ester film, a production method of the cellulose ester film, and a polarizer using the cellulose ester film, which cellulose ester film excels in optical properties, dimensional stability, transparency, flatness, and resistance to frame-like white patches.
PTL 4 discloses the use of a cellulose acylate which has a degree of acyl substitution at the 2-position and the 3-position in total of 1.70 or more and 1.90 or less and has a degree of acyl substitution at the 6-position of from 0.88 or more, namely, has a total degree of substitution of from 2.58 to 2.78, in order to obtain a cellulose acylate solution showing excellent stability with time and having a low viscosity at dope concentrations within a practically usable range. PTL 4 also discloses, in Example 1, a cellulose acetate having a degree of substitution at the 2- and 3-positions of 1.88 and a degree of substitution at the 6-position of 0.89, namely, having a total degree of substitution of 2.77.
A technique disclosed in PTL 5 has been made to provide an optical film, a production method thereof, and a polarizing film using the same, which optical film is useful as a protective film for a polarizing film for use in liquid crystal display devices and is composed of a cellulose ester film, in which the optical film has a high modulus of elasticity both in machine and transverse directions, whereby suppresses the shrinkage of the polarizing film, and preventing the polarizing film from peeling off from a liquid crystal cell due to the shrinkage of the polarizing film. PTL 5 describes that an optical film composed of a cellulose ester film is produced through a solution casting filming process. Specifically, PTL 5 discloses a technique including the steps of preparing a solution of a cellulose ester having a molecular weight distribution (Mw/Mn) as a ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of 1.4 to 3.0; casting the cellulose ester solution onto a support to form a web; separating the film (web) from the support; and, while drying the film, stretching the film to a draw ratio of 1.1 to 1.5 times simultaneously both in a film transporting direction (machine direction (MD)) and in a direction (transverse direction (TD)) being in the film plane and being perpendicular to the film transporting direction during when the amount of residual solvent in the film be from 10% to 100%.
A technique disclosed in PTL 6 has been made to provide a cellulose ester film which is suitable as a polarizer-protective film in a liquid crystal display device (LCD), has a satisfactory distribution of orientation angle in a transverse direction of the cellulose ester film, does not suffer from so-called banding defects (banding thickness variation) upon separation from the support, has high quality, is peelable stably, is very advantageous in cost, and is inexpensive. PTL 6 describes that, of cellulose ester films each produced by a solution casting filming process and composed of two or more different cellulose esters, preferred is a cellulose ester film containing a cellulose ester derived from wood pulp and having a molecular weight distribution Mw/Mn of from 1.8 to 3.0, in which the content of this cellulose ester is 50 percent by weight or more based on the total weight of cellulose esters constituting the film. PTL 6 describes that the ratio of a wood pulp-derived cellulose triacetate to a cotton linter-derived cellulose triacetate in cellulose triacetates was set to be 100/0 in Examples 1 and 4, to be 80/20 in Examples 2, 5, and 7, and to be 50/50 in Examples 3 and 6; and that the molecular weight distribution (Mw/Mn) of the wood pulp-derived cellulose triacetate was set to be 1.8 in Examples 1 to 3, to be 3.0 in Examples 4 to 6, and to be 2.3 in Example 7, whereas the molecular weight distribution (Mw/Mn) of the cotton linter-derived cellulose triacetate was set to be constant, i.e., 3.5 in all the examples.
PTL 7 discloses a cellulose ester film having a molecular weight distribution (Mw/Mn) of from 1.0 to 5.0 (claim 5) and describes that the cellulose ester preferably substantially a cellulose triacetate (Paragraph 0056). PTL 7 discloses films each containing a cellulose triacetate and having a molecular weight distribution (Mw/Mn) of 3.5 in Examples 6 and 9, but fails to investigate or specify other parameters such as thickness-direction retardation Rth.
PTL 8 discloses a retardation film and a production method thereof, which retardation film shows less variation in retardation properties even when it has a small thickness and is used for a long time or used in a varying environment. PTL 8 also discloses a retardation film which shows a wide viewing angle when used in a liquid crystal image display device and which shows a satisfactory viewing angle even when used over a long term. Specifically, the retardation film disclosed in PTL 8 has an in-plane retardation Ro of from 30 to 200 nm and a thickness-direction retardation Rt of from 70 to 400 nm, which film is formed from a cellulose ester having a total degree of acyl substitution of from 2.40 to 2.80 and an unsubstitution degree of hydroxyl groups at the 6-position of from 0.15 to 0.42. However, PTL 8 does not consider the molecular weight distribution Mw/Mn of the cellulose ester and merely provides retardation films each having a thickness-direction retardation Rt of at most 155 nm or less in the working examples.
PTL 9 discloses the production of a 6-position highly acetylated cellulose diacetate which is useful typically as a material for cellulose acylates of different acyl groups, which cellulose acylates have a high total degree of acyl substitution and are usable typically as materials typically for photographic materials and optical materials. This literature discloses a 6-position highly acetylated cellulose diacetate which is a cellulose diacetate having a 6-percent viscosity of 40 to 600 mPa·s and having a total degree of acetyl substitution of DSt and a degree of acetyl substitution at the 6-position of DS6, in which DSt and DS6 satisfy following Relational Expressions (1) and (2): 2.0≦DSt<2.6; 0.400≧(DS6/DSt)≧0.531−0.088×DSt. An object of the technique disclosed in this literature is to provide a cellulose diacetate which is highly acetylated at the 6-position, has a not-so-high total degree of acetyl substitution so as to have a certain margin of introducing other acyl groups than acetyl group, and has a relatively high molecular weight. The literature also discloses a process for producing such a cellulose acetate, which process includes the steps of reacting a cellulose with an acetylating agent in a solvent in the presence of a catalyst to give a cellulose triacetate having a total degree of acetyl substitution of 2.6 or more; and hydrolyzing the cellulose triacetate in acetic acid in the presence of an acetylation catalyst in an amount of from 0.56 to 8.44 parts by weight per 100 parts by weight of the cellulose triacetate and in the presence of water in an amount of 22 percent by mole or more and less than 50 percent by mole relative to the amount of the acetic acid at a temperature of from 40° C. to 90° C., to give a cellulose diacetate being highly acetylated at the 6-position. The literature describes that the 6-position highly acetylated cellulose diacetate is preferably a cellulose diacetate having a uniformly distributed total degree of acetyl substitution; that the uniformity of the total degree of acetyl substitution may be determined based on, as an index, the magnification of half height width of a maximum peak in an intermolecular substitution distribution curve or in an intermolecular acetylation distribution curve of the cellulose diacetate; and that the half height width of a maximum peak in an intermolecular substitution distribution curve of the cellulose diacetate is preferably 0.150 or less, more preferably 0.140 or less, and particularly preferably 0.130 or less.
An object of the technique disclosed in Citation 9 is to provide a cellulose acetate material which has a not-so-high total degree of acetyl substitution so as to have a certain margin of introducing other acyl groups than acetyl group, because, when a substituent having carbon atoms in a larger number than that of acetyl group is introduced into an original cellulose acetate, the resulting cellulose acetate can have higher stretchability. Citation 9, however, fails to disclose the ratio (Mw/Mn) of the weight-average molecular weight Mw to the number-average molecular weight Mn, and neither describes nor indicates the optimization of the ratio (Mw/Mn) of the weight-average molecular weight Mw to the number-average molecular weight Mn to obtain high stretchability.
Fundamental principles of processes for preparing cellulose acetates are described in Non-Patent Literature (NPL) 1. A representative preparation process is a liquid phase acetylation process using acetic anhydride (as an acetyl group donor), acetic acid (as a solvent), and sulfuric acid (as a catalyst). Specifically, a material cellulose such as wood pulp is pretreated (activated) with an adequate amount of acetic acid, and the pretreated material is added to a previously cooled acetylation mixture to convert into an acetic acid ester to thereby give a cellulose acetate. The acetylation mixture generally contains acetic acid as a solvent, acetic anhydride as an acetyl group donor (esterifying agent), and sulfuric acid as a catalyst. The acetic anhydride is generally used in a stoichiometrically excess amount to the total amount of the cellulose as a reactant and water in the system. After the completion of the acetylation reaction, an aqueous solution of a neutralizer is added for the hydrolysis of excess acetic anhydride remained in the system and for the neutralization of part of the esterification catalyst. Exemplary neutralizers herein include carbonates, acetates, and oxides of sodium, potassium, calcium, magnesium, iron, aluminum, zinc, and ammonium. According to known processes, the prepared cellulose acetate is ripened (aged) by maintaining the same at a temperature of from 50° C. to 90° C. in the presence of a small amount of an acetylation catalyst (generally, the residual sulfuric acid) to convert into a cellulose acetate having a desired degree of acetyl substitution and a desired degree of polymerization. At the time when such a desired cellulose acetate is obtained, the residual catalyst in the system is fully neutralized with a neutralizer as mentioned above, or, without neutralizing the residual catalyst, the cellulose acetate solution is poured into water or diluted acetic acid, or water or diluted acetic acid is poured into the cellulose acetate solution, to separate the cellulose acetate, and the separated cellulose acetate is washed and stabilized to thereby give a product cellulose acetate.
In the activation step (or pretreatment step), a cellulose is activated by treating with an acetylation solvent (solvent used in the acetylation step). Acetic acid is generally used as the acetylation solvent, but exemplary acetylation solvents usable herein further include other solvents than acetic acid, such as methylene chloride; and a solvent mixture of acetic acid with another solvent than acetic acid, such as methylene chloride. The material cellulose is generally supplied in the form of a sheet, and the sheet-like material cellulose is broken into pieces in dry manner and then subjected to the activation treatment (or pretreatment). A strong acid such as sulfuric acid may be added to the acetylation solvent for use in the activation step. However, treatment with an acetylation solvent containing a large amount of a strong acid may accelerate the depolymerization of the cellulose and may cause the cellulose to have a lower degree of polymerization. Typically, the amount of a strong acid (sulfuric acid) added in the pretreatment step according to a common technique is about 0.1 to 0.5 part by weight per 100 parts by weight of the material cellulose. It has been revealed that the use of a strong acid (sulfuric acid) in an amount of 0.5 part by weight or more per 100 parts by weight of the material cellulose in the pretreatment step causes the cellulose to have a low molecular weight (NPL 2). According to the known technique, the activation step is performed for a time (treatment time) of typically from 10 to 180 minutes, and preferably from 20 to 120 minutes.
According to the known technique, an acetylation reaction in the acetylation step may be performed for a time (total acetylation time) of typically from 20 minutes to 36 hours, and preferably from 30 minutes to 20 hours, while the acetylation time may vary depending typically on the reaction temperature. The acetylation is particularly preferably carried out at least at temperatures of from 30° C. to 50° C. for about 30 minutes to 180 minutes, and preferably about 50 minutes to 150 minutes. Additionally, in the known technique, the time of terminating the acetylation reaction is not recognized as significantly affecting the properties of the resulting cellulose acetate and is often determined for reasons of the process, typified by limitation by the amount of the neutralizer fed through a feed piping.
In the synthesis of a cellulose acetate, an acetylation reaction of cellulose and a cleaving reaction (depolymerization) of β-glucosidic bonds constituting the molecular frame of the cellulose simultaneously proceed in parallel with each other. The depolymerization reaction of cellulose can be considered as a random reaction and follows a most probable distribution. Accordingly, when the depolymerization proceeds to some extent in the synthesis of a cellulose acetate, the molecular weight distribution of the cellulose acetate approaches 2, as is the case with a random polymer having a sufficiently increased molecular weight which has a molecular weight distribution of 2. As is demonstrated above, the esterification and hydrolysis of a cellulose acetate should be performed in the shortest possible time to allow the cellulose acetate to maintain a large molecular weight distribution when the cellulose acetate is a cellulose diacetate.
In contrast, when a cellulose acetate has a uniform chemical composition in the synthesis thereof, it means that the material cellulose is sufficiently reacted both in the cellulose esterification step and in the hydrolysis step of the resulting cellulose ester to allow the cellulose acetate to have a uniform chemical composition. Accordingly, the esterification and hydrolysis of a cellulose acetate should be performed for a sufficiently long time in order to allow the cellulose acetate to have a uniform chemical composition. Thus, a uniform chemical composition and a ununiform molecular weight distribution are incompatible requirements with each other in the cellulose acetate synthesis, and compatibility between these properties has not yet been achieved.